is the leak before the sensor? if an exhaust leak before your sensor didn't change your reading even a little bit then i would go ahead and say your sensor is bad. it should have changed a little bit.

but yes, pull your sensor out and just set it on the ground or your fender or somthing and turn your car on. make sure it goes to full lean, and then put a little bit of gas on a rag or paper towel (don't go crazy with the gas lol). then set the sensor in the rag and check the readings. if it doesn't go and stay at full rich i would go ahead and say the sensor is bad.